The present invention relates to a curable composition for build-up which comprises a specific epoxy compound. More particularly, it relates to a curable composition for build-up having a high glass transition temperature, excellent electrical characteristics, mechanical strength, and a small water absorption, which is characterized by comprising 2,2-bis(3,4-epoxycyclohexyl)propane (hereinafter referred to as DCA) and a polyepoxy compound having a polyhydric phenol skeleton in a specific range of ratio.
A full additive process for producing a high-density printed circuit board, in which a plating resist finally becomes an insulating layer between wires, which makes it possible to prevent positional deviations in superposing printed circuit boards and is suitable for building up printed circuit boards having high aspect ratio wiring.
The plating resist remaining on the printed circuit board as an insulating layer is required to have a high glass transition temperature, a high volume resistivity, high mechanical characteristics, and a low water absorption so as to cope with the heat generation by the highly integrated circuit, reduction of the insulating layer thickness, and reduction in adhesive strength between the conductor layer and the insulating layer.
It is widely known to use an epoxy as an insulating layer. German Patent 1099733 discloses curing of DCA with an acid anhydride, and Japanese Patent Application Laid-Open No. 225580/90 describes curing of DCA with a initiator for cationic photopolymerization. However, a cured product obtained by curing DCA with phthalic anhydride or a photopolymerization initiator is brittle and impractical.
A cured product obtained from an epoxy compound having a polyhydric phenol skeleton, such as bisphenol A diglycidyl ether, and an acid anhydride is impractical for build-up due to its low glass transition temperature.
Accordingly, an object of the present invention is to provide a curable composition having a high glass transition temperature, excellent electrical characteristics, mechanical strength, and a low water absorption.
The present inventors have conducted extensive investigation and found as a result that a curable composition comprising a mixture of DCA and an epoxy compound having a polyhydric phenol skeleton at a specific ratio accomplishes the above-described object.
Having been completed based on the above finding, the present invention provides a curable composition for build-up which comprises (A) a bisepoxy compound represented by formula (I) shown below and (B) a polyepoxy compound having a polyhydric phenol skeleton at a weight ratio (A)/(B) of 3/97 to 20/80. 